The present invention relates to a lubricant blend for magnetic wires that are employed in devices, such as compressors, which employ a refrigerant. More particularly, the present invention relates to a lubricant blend which imparts coefficient of friction values to magnetic wire comparable to that in which a paraffin lubricant and which is compatible with HFC (hydrofluorocarbons) refrigerants.
In the manufacturing of electric motors, magnetic wire is wrapped in the motor in a tight coil. In general, tighter and closer winding of the wire allows more wire to be inserted in a motor of a given size. This generally allows a greater operating efficiency, or space factor. There is an ever-increasing need for motors of even higher efficiency to satisfy energy conservation goals, particularly with hermetic motors used in air conditioners and refrigerators. To meet this end, even more magnetic wire is installed in a motor to achieve a higher space factor.
Prior to assembly, wires are lubricated to minimize friction. This has been accomplished in the past by coating the wire with lubricants such as liquid paraffin. Paraffin has established itself as a preferred external lubricant on magnetic wire to promote windability and insertion in electric motor stators, particularly when used with conventional chlorofluorocarbon (CFC), hydrochlorofluorocarbons (HCFCs), or hydrocarbon (HC) refrigerants. The mechanism by which paraffin promotes windability and insertion is its effectiveness in reducing the wire-to-wire coefficient of friction (COF).
Recent mandates to eliminate CFC refrigerants and other ozone depleting substances require refrigeration compressor manufacturers to convert to non-ozone depleting refrigerants such as hydrofluorocarbons. One example of an HFC is 1,1,1,2-tetrafluoroethane (R134a). This refrigerant exhibits different chemical compatibilities than conventional CFC refrigerants. See, for example, S. G. Sundaresan, W. R. Finkenstadt, Polyalkylene Glycol and Polyolester Lubricant Candidates for Use With HFC-134a in Refrigeration Compressors, ASHRAE Transactions 1992, Vol. 98, Pt. 1, AN-92-5-3. As a result, compatibility investigations have determined that a variety of process fluids conventionally used in compressor manufacturing and operation can no longer be used with these non-ozone depleting refrigerants in view of their differing chemical compatibilities.
Testing of compressors with R134a indicates that serious problems may occur if paraffin is used as an external lubricant to promote the windability of magnetic wire in motor stators. Because paraffin is insoluble in R134a, residues of paraffin can detach from the magnetic wire and get transported to and deposited in critical orifices in the compressor, and thus, block the opening of the compressor valve, the refrigerant expansion valve, or other capillaries. This often leads to compressor failure. Consequently, the use of paraffin as a magnetic wire lubricant in conjunction with the use of non-ozone depleting refrigerants, such as R134a, for example, is undesirable.
Alternate external lubricants have been developed in conjunction with internal lubricants. These lubricants are generally baked into the wire coating to further reduce wire-to-wire COF. These external lubricants are generally based on esters of fatty acids and/or pentaerythritol. Although effective, these lubricants can be quite costly, or require the use of uneconomical and/or hazardous solvents for application.
There is also a need to minimize emissions from manufacturing plants. Conventional lubricants such as paraffin are dissolved in organic solvents, which not only constitute a flammability and potential health hazard, but also increase total air emissions. Water-based lubricants have not been successfully implemented due to ineffective lubrication of the wire or poor processing characteristics, such as poor wettability of applicator felts. As a result, it is also desirable to develop a water-based lubricant that can decrease wire-wire COF, while also being soluble in non-ozone depleting refrigerants, such as R134a, over a wide temperature range.
Thus, there is a particular need for a lubricant blend that imparts a coefficient of friction (COF) value to magnetic wire that is comparable to a magnetic wire to which a paraffin lubricant is applied, but which unlike paraffin lubricant is compatible with non-CFC containing refrigerants. Furthermore, there is a need for a lubricant blend has good lubricity, good processing characteristics and good wettability characteristics in addition to a good solubility in refrigerants.
In a first aspect, the present invention relates to a lubricant blend comprising an emulsion comprising:
(i) an organic phase comprising at least one lubricant, a solvent in which the at least one lubricant is soluble and optionally at least one surfactant, wherein which promote magnet wire lubricity, wherein the at least one lubricant has a solubility in an HFC refrigerant of at least about 0.2% by weight at 0xc2x0 C. and greater than 0.1% in a blend of 2 parts HFC refrigerant and 1 part of a synthetic oil at xe2x88x9220xc2x0 C. (referred to later as xe2x80x9cspecial solubility characteristicsxe2x80x9d); and
(ii) an aqueous phase comprising at least one surfactant which forms an emulsion between the organic phase and the aqueous phase and exhibiting the same solubility as the at least one lubricant.
The synthetic oil is a refrigeration grade synthetic oil, such as a polyolester oil or a polyalkylene glycol oil, with suitable solubility characteristics in an HFC refrigerant.
Preferred lubricants include fatty acid esters, such as alkyl stearyl esters. Especially preferred as the lubricant is methyl stearate.
The surfactant which forms an emulsion with the lubricant is preferably a mixture of a non-ionic surfactant having a lower hydophilic/lipophilic balance (HLB) of between 1 and 7 and a non-ionic surfactant having a higher HLB of between 7 and 12.
The solvent is generally a hydrophobic solvent and more preferably a hydrocarbon.
In a second aspect, the invention relates to a method for making a lubricant blend comprising the steps of:
dissolving at least one lubricant and optionally one or more surfactants having an HLB value of 1 to 7, both of which have a solubility in a non-CFC containing refrigerant of at least 0.2% at 0xc2x0 C. into a hydrophobic solvent and into a first surfactant having an HLB value of 1 to 7 to form a first organic phase mixture;
admixing the organic phase mixture with an aqueous solution comprising 0.5 to 2% of a second surfactant having an HLB value of 7 to 12, with a solubility in a HFC refrigerant of at least 0.2% at 0xc2x0 C. and greater than 0.1% in 2 parts HFC refrigerant/1 part HFC soluble synthetic oil at xe2x88x9220xc2x0 C., the proportion of the organic phase mixture and the aqueous phase mixture being such that the ratio of the hydrophilic and hydrophobic surfactant is 0.5 to 2;
mixing the organic and aqueous phases to form a stable emulsion.
In a preferred embodiment, the organic and aqueous phases are admixed by sonication.
In a third aspect, the invention relates to a compressor comprising an electric motor including a magnetic wire wrapped in a coil, wherein the magnetic wire is coated with the above-described lubricant blend.
In its most preferred aspect the lubricant blend comprises methyl stearate and polyethylene-polypropylene glycol copolymers in a suitable solvent. The lubricant blend may be water based and includes methyl stearate and hydrophilic and hydrophobic surfactants comprised of polyethylene glycol polymers, polypropylene glycol polymers, and copolymers thereof. The components are dissolved in a hydrophobic solvent and emulsified in water. Installation of the magnetic wire is eased by the wire-wire friction reduction achieved by these blends.
Unlike paraffin, the lubricant blends of the invention are soluble in non-ozone depleting refrigerants such as the HFCs and more particularly, 1,1,1,2-tetrafluoroethane (R134a), and standard compressor lubricants used in conjunction with R134a. Consequently, there is no danger of lubricant particles clogging critical orifices of the compressor during operation.